WD-repeat proteins belong to a large family of structurally related proteins, members of which have diverse functions such as cell cycle regulation, transcription, chromatin organization and protein trafficking [1,2]. These proteins provide a platform for protein-protein interactions. WDR13 protein is a member of this family and localizes to the nucleus [3]. Wdr13 gene is highly conserved in vertebrates and is expressed in most of the tissues [4], the highest level of expression being in pancreas, brain, testis and ovaries. This gene is located on the X-chromosome at locus Xp11.23 and XA1.1 in human and mouse, respectively. In human, X chromosomal deletions including this gene have been associated with mental retardation, obesity and xeroderma [5,6,7,8]. Several WD-repeat proteins have been identified which express in pancreatic beta cells and have roles in beta cell proliferation [9,10].
The beta cell mass is regulated by the balance between neogenesis/proliferation and apoptosis/necrosis. In mice, differentiation of islet precursor and expansion are responsible for beta cell neogenesis until the first week of life [11,12]. Thereafter, expansion of existing beta cells is the main source of newly formed beta cells [13,14]. In pathological conditions there can be alpha to beta cell trans-differentiation [15]. Various cell cycle regulators have been identified which have role in pancreatic beta cell proliferation [16]. Cell cycle progression in pancreatic islet is controlled by cyclins, cyclin dependent kinases (CDKs), cyclin dependent kinase inhibitors and hormones, namely: androgen and estrogen [16,17]. Estrogen enhances pancreatic beta cell mass by protecting them from apoptosis [17,18,19,20].
Pancreatic islet mass, insulin production and body weight are inter-related [12,21]. Insulin levels have been positively correlated with obesity in humans [22] and rodents [23]. Generally, obesity leads to higher demand for insulin production and the same is met by the increase in beta cell mass. Obesity is also a major risk factor for the onset of peripheral insulin resistance [24]. Insulin resistance leads to further higher demand for insulin from beta cells triggering beta cell failure. This leads to beta cell survival defects, insufficient beta cell mass and deterioration of key beta cell function such as glucose stimulated insulin secretion, and ultimately type 2 diabetes. Thus, the mass of insulin producing beta cells changes dynamically according to the metabolic conditions [25,26]. Alternatively, obesity may be a consequence of higher insulin levels [27,28,29] as insulin has stimulatory effect on adipogenesis by increasing the lipid accumulation in adipocytes [30,31,32]. Insulin is also involved in adipocyte survival [33]. Adipose tissue-specific insulin receptor knockout protects against obesity, emphasizing that insulin signaling to adipocytes is important for development of obesity [31]. Hyper insulin secretion in MOR-1 opioid receptor knockout mice results in more body weight gain with age [29] whereas CHOP knockout mice become obese by increasing insulin secretion although without affecting glucose tolerance [34].
To understand the in vivo role of Wdr13 gene, we have created a mouse strain lacking this gene and show that these mice have higher pancreatic islet mass as a result of higher beta cell proliferation, develop hyperinsulinemia and mild obesity. We have also identified several interacting partners of WDR13 protein and provide evidence that this protein may be working as a repressor of transcription.